Chabazite-type zeolite is a zeolite which has a three-dimensional pore structure configured constructed from 8-membered oxygen rings of 3.8×3.8 angstrom, and is designated and classified with the structure type code of CHA, as a zeolite identified the detailed crystal structure, by the International Zeolite Association (Non-Patent Document 1).
Chabazite-type zeolite is known as the naturally occurring zeolite, and typically has the composition of Ca62+[Si24Al12O72] (Non-Patent Document 2). As examples of synthetic zeolite, the chabazite-type zeolite with a SiO2/Al2O3 molar ratio of 3.45-4.9 has been disclosed as Zeolite D in Patent Document 1, and as Zeolite R in Patent Document 2. As a typical method of synthesis, the method in which a chabazite-type zeolite is crystallized from Y-type zeolite as a raw material under hydrothermal conditions is disclosed in Patent Document 3.
In Patent Document 4 and Patent Document 5, the so-called high-silica chabazite-type zeolite with a SiO2/Al2O3 molar ratio of 5-50, which is designated as SSZ-13, and its method of synthesis are disclosed.
In Patent Document 6, the chabazite-type zeolite with a SiO2/Al2O3 molar ratio of 20-50 and a crystal diameter of 0.5 μm or less is disclosed as SSZ-62.
Furthermore, the possibility to synthesize a chabazite-type zeolite with a SiO2/Al2O3 molar ratio of 50 or more is respectively disclosed in Patent Document 7 and Non-Patent Document 3 with respect to the method to produce it with fluorine, and in Patent Document 8 with respect to the method without fluorine.
In recent years, a copper loaded chabazite-type zeolite has particularly attracted attention as a selective reduction catalyst of NOx in automotive exhaust gas.
As an example of copper loaded chabazite-type zeolite, there has been disclosure of a catalyst loading copper on SSZ-62 (Patent Document 6), as well as a copper loaded catalyst wherein the SiO2/Al2O3 molar ratio is greater than approximately 15, and the atomic ratio of copper to aluminum is in a range exceeding approximately 0.25 (Patent Document 9).
Moreover, Patent Document 10 discloses a catalyst consisting of chabazite-type zeolite wherein the SiO2/Al2O3 molar ratio is 15-50, and the average particle size is 1.5 μm or more.
There has also been disclosure of a chabazite-type zeolite which has a SiO2/Al2O3 molar ratio of less than 15, and which can be used as a NOx removal catalyst (Patent Document 11). Furthermore, in Patent Document 11, it is shown that a chabazite-type zeolite with a SiO2/Al2O3 molar ratio of less than 10 is desirable for use in the thermal condition at 700° C.
However, these chabazite-type zeolite catalysts still have insufficient NOx reduction rates in low-temperature region after durability treatment in a high-temperature steam atmosphere (also referred to as “hydrothermal durability treatment”). Consequently, a NOx reductive removal catalyst with higher performance is desired.
Thus, chabazite-type zeolites are anticipated to be utilized in a variety of applications, particularly as an adsorbent or a catalysis support. However, for industrial use, it must have sufficient ion exchange capacity and solid acidity, as well as durability for an adsorbent or catalysis support. For example, for use in an adsorption-desorption process involving a thermal regeneration step, the zeolites are required not to decline in the adsorption performance even when repeatedly heated, or zeolite catalysts used in exhaust gas purification are required to have the thermal durability in order to retain the catalytic performance under high temperature. Furthermore, for use in catalysts and adsorbents, the particle size distribution of zeolites must be in the appropriate range, because zeolites are used in the form of an extruded product or a coated one onto honeycomb substrate. Consequently, a previously unobtainable chabazite-type zeolite is required which has better durability and heat resistance, which has a high NOx reduction rate in low-temperature region after hydrothermal durability treatment, and which also has a controlled particle size distribution.